Description of problem:
When syncing channel from CDN using the cdn-sync command, there is possibility of missing RPM in the CDN. Cdn-sync should log explicit error in its log in such occasion, and exit with non-zero exit code. Currently it doesn't report any specific error in the log, and exits with 0 (success) exit code.

Version-Release number of selected component (if applicable):
spacewalk-backend-cdn-2.5.3-32

How reproducible:
Issue is present only when there is missing RPM file in the CDN (happens sometimes on the code-stage). As of today, there is missing libkadm5-1.14.1-26.el7-0.i686 in the rhel-x86_64-server-7 channel in the code-stage.

Steps to Reproduce:
1. cdn-sync -c rhel-x86_64-server-7

Actual results:

# cdn-sync -c rhel-x86_64-server-7
08:14:13 ======================================
08:14:13 | Channel: rhel-x86_64-server-7
08:14:13 ======================================
08:14:13 Sync of channel started.
08:14:13 Repo URL: https://cdn.stage.redhat.com/content/dist/rhel/server/7/7Server/x86_64/os
<...>
08:33:20 6307/13875 : libkadm5-1.14.1-26.el7-0.i686
08:33:21 failure: Packages/libkadm5-1.14.1-26.el7.i686.rpm from content_dist_rhel_server_7_7Server_x86_64_os: [Errno 256] No more mirrors to try.
08:33:21 ERROR: failure: Packages/libkadm5-1.14.1-26.el7.i686.rpm from content_dist_rhel_server_7_7Server_x86_64_os: [Errno 256] No more mirrors to try.
08:33:21 Repo URL: https://cdn.stage.redhat.com/content/dist/rhel/server/7/7.1/x86_64/kickstart
08:33:34 Packages in repo:              4371
<...>
08:35:59 Linking packages to channel.
08:36:23 ERROR: Could not find object [<spacewalk.server.importlib.importLib.IncompletePackage instance; attributes={'package_size': None, 'package_arch_id': 104, 'name': 'libkadm5', 'checksum_list': None, 'md5sum': None, 'org_id': None, 'epoch': '0', 'checksums': {'sha1': '553b168dc506d33f684f3a0c36f6054a3fa253b9'}, 'channels': {101: 'rhel-x86_64-server-7'}, 'nevra_id': 8689, 'package_id': None, 'last_modified': None, 'name_id': 3597, 'version': '1.14.1', 'checksum': '553b168dc506d33f684f3a0c36f6054a3fa253b9', 'release': '26.el7', 'checksum_type': 'sha1', 'arch': 'i686', 'evr_id': 5310, 'checksum_id': 773956}] in table rhnPackage
08:36:23 Sync of channel completed in 0:09:53.
08:36:23 Total time: 0:09:53

[root@hp-dl360g6-01 ~]# echo $?
0


[root@hp-dl360g6-01 ~]# tail /var/log/rhn/cdnsync/rhel-x86_64-server-7.log 
2016/11/22 08:35:59 -04:00 4298/4305 : zip-3.0-10.el7.x86_64
2016/11/22 08:35:59 -04:00 4299/4305 : zlib-1.2.7-13.el7.i686
2016/11/22 08:35:59 -04:00 4300/4305 : zlib-1.2.7-13.el7.x86_64
2016/11/22 08:35:59 -04:00 4301/4305 : zlib-devel-1.2.7-13.el7.x86_64
2016/11/22 08:35:59 -04:00 4302/4305 : zlib-devel-1.2.7-13.el7.i686
2016/11/22 08:35:59 -04:00 4303/4305 : zsh-5.0.2-7.el7.x86_64
2016/11/22 08:35:59 -04:00 4304/4305 : zziplib-0.13.62-5.el7.x86_64
2016/11/22 08:35:59 -04:00 4305/4305 : zziplib-0.13.62-5.el7.i686
2016/11/22 08:35:59 -04:00 Linking packages to channel.
2016/11/22 08:36:23 -04:00 Sync of channel completed in 0:09:53.

Expected results:
Cdn-sync should report the error more explicitly (and maybe continue the sync, if possible, not sure about that). Currently there is no indication of error in the log. Exit code should be something different than 0.

Additional info: